Arsenic enrichment and mobilization in the Holocene alluvial aquifers of the Chapai-Nawabganj district, Bangladesh: A geochemical and statistical study

Core sediments from three boreholes (to a depth of 50m) and groundwater from 20 As-enriched water wells were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analyses. Multivariate statistical analyses including factor analysis, cluster analysis and multidimensional scaling were applied to the hydrogeochemical data. The results show that a few factors adequately represent the traits that define water chemistry. Sodium, Cl, SO₄, total dissolved solids (TDS), and electrical conductivity (EC) are grouped under the first factor representing the salinity sources of waters. The second factor, represented by As and Mn, is related to As mobilization processes. The third factor of Fe and alkalinity is strongly influenced by bacterial Fe(III) reduction which would raise both Fe and HCO3- concentrations in water. The fourth factor of Ca and Mg reflects the hardness of the Ca-HCO₃ type of groundwater, which is confirmed by the hydrochemical facies analysis. Cluster analysis leads to the formulation of four water types including highly, moderately, and slightly As-enriched groundwater as well as groundwater with elevated SO42-, from anthropogenic sources. Multivariate analyses of the geochemical parameters suggest that Fe- and Mn-oxyhydroxides and mineral phases of phyllosilicates (e.g., biotite) are the main hosts of As in the sediments. Statistical analysis also shows that As is closely associated with Fe and Mn in sediments while As is positively correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing As into the groundwater.